Cyanobacteria are those organisms formerly known as blue‐green algae. These members of the Monera kingdom are photosynthetic. Most are found in the soil and in freshwater and saltwater environments. The majority of species are unicellular, but some may form filaments. Like the other bacteria, all cyanobacteria are prokaryotes.

Cyanobacteria, which are autotrophic, serve as important fixers of nitrogen in food chains. In addition, cyanobacteria, a key component of the plankton found in the oceans and seas, produce a major share of the oxygen present in the atmosphere, while also serving as food for fish. Some species of cyanobacteria coexist with fungi to form lichens.

Cyanobacteria have played an important role in the development of the earth. Scientists believe that they were among the first photosynthetic organisms to occur on the earth's surface. Beginning about 2 billion years ago, the oxygen produced by cyanobacteria enriched the earth's atmosphere and converted it to its modern form. This conversion made possible all life‐forms that use oxygen in their metabolisms.